The invention relates to an improved method for the continuous refining of magnesium by the precipitation of impurities in the form of sludge and to a refining furnace for performance of the method.
Most magnesium-refining today is done discontinuously in crucibles placed under lids in suitable electric furnaces. After a certain period of time impurities are separated from the magnesium by settling as a sludge in the bottom of the crucible. The refined magnesium collects in the upper part of the crucible and is decanted, and the crucible then is cleaned of sludge prior to the next use. This method is characterized by low productivity, high energy consumption and metal losses caused by metal oxidation. Furthermore, this method results in unpleasant working conditions for the operators exposed to heat and gases from the melt.
There is a known construction principle for a continuously operating refining furnace. Such a furnace comprises a rectangular refractory lined body, divided by means of vertical partition walls into several chambers. Raw magnesium is continuously charged into the first chamber and passes through openings in the partition walls, provided at a level corresponding to the metal level in the furnace, and the metal is transferred successively from one chamber to the next. The sludge and the salt melt one gradually precipitated in the individual chambers and accumulated in the bottoms of the chambers. The purified magnesium is discharged from the last (successive) chamber. The furnace is provided with a lid which has openings for charging/discharging of magnesium and for the removal of the sludge from the individual chambers. A protective gas is fed into the chambers in order to avoid metal oxidation.
However, in spite of the obvious advantages compared with the discontinuous crucible refining, even this construction is not quite satisfactory. The capacity of such furnaces is limited and the accumulated sludge has to be removed individually from each chamber. The furnace therefore has to be regularly shut down for the discharge of the sludge. Through openings in the furnace lid, both the protective gas and the fumes from the melt are released to the atmosphere, and air entering the chambers oxidizes some of the magnesium. Besides, the sludge discharge operation results in a considerable heat loss from the furnace.
U.S. Pat. No. 3,882,261 describes another type of furnace for continuously refining magnesium. The furnace, which is cylindrically shaped, is divided by means of vertical partition walls into a central chamber and peripheral chambers surrounding the central chamber. The partition walls between the peripheral chambers are provided with openings for the transfer of the charged metal from one chamber to the next in the direction of the refining process, with the gradual precipitation of sludge in the chambers. The central chamber, which is closed at its upper part by the furnace lid and separated in this way from the peripheral chambers, receives only the bath melt and no magnesium. The furnace bottom is provided with sloped walls enabling the sludge from the peripheral chambers to accumulate in the bottom of the central chamber.
This construction theoretically provides a furnace with a centralized sludge discharge where it is not necessary to interrupt the refining process, since the peripheral chambers with magnesium remain closed during the removal of sludge. However, there is a strong probability that a part of the sludge will also accumulate in the peripheral chambers, and such sludge has to be periodically removed. Furthermore the patent states that in order to achieve a productivity of 80-100 t/day the furnace capacity has to be 30-35 m.sup.3.
The relatively high current velocities between the peripheral chambers make it necessary to provide such large furnace volume to achieve a sufficient treatment time in order to obtain the required purification grade of the metal. The furnace is very deep, which is unfavourable both from the construction point of view and with regard to the insertion of the device for removal of the sludge. Besides the high capital and operating costs, the furnace represents a safety risk for the operators during the possible leakage of such a mass of liquid magnesium. Accordingly, the object of the present invention is to overcome the above mentioned difficulties.